Two-cycle internal combustion engine

ABSTRACT

A two-cycle internal combustion engine comprises a cylinder head having an intake port and exhaust port opened to a combustion chamber, to which compressed fresh air is introduced via an intake valve. The intake and exhaust valves are operated in response to a crank angle. A pulsation pressure of exhaust gas in the exhaust port is substantially restrained during an idling or light load running condition. The exhaust valve is opened earlier than the intake valve when the speed of the downward movement of the piston is relatively high, such that a part of exhaust gas in the exhaust port flows back to the combustion chamber in that condition. An exhaust gas swirl rotating around a cylinder axis is formed when the exhaust gas flows back and the fresh air is slowly introduced onto the exhaust gas swirl in that condition.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a two-cycle internal combustion engine and,more particularly, to a two-cycle engine having intake and exhaustvalves which are operated in response to a crank angle.

2. Description of the Related Art

Japanese Examined Patent Publication No. 60-5770 discloses anopen-chamber type two-cycle engine having intake and exhaust valves. Inthis two-cycle engine, when a piston is at bottom dead center, bothvalves are opened. Fresh air is introduced into a combustion chamber viathe intake valve and along the cylinder wall so as to form a verticalloop flow. The boundary between the fresh air and the exhaust gas movesfrom a position in the vicinity of the intake valve to the center of thecylinder, and then to a position in the vicinity of the exhaust valve,so that the fresh air is substituted for the exhaust gas over the wholeregion in the combustion chamber.

In this known two-cycle engine, there are no particular problems when itis running in a heavy load condition. However, in an idling or lightload condition, the flow of fresh air is so small that a large amount ofexhaust gas remains in the cylinder, which prevents the fresh air fromconcentrating around the cylinder head having an ignition spark plug.This is because, according to the vertical loop flow, the main part ofthe fresh air moves to the lower part of the cylinder. Therefore,initial burning by the spark plug is disturbed and/or the generation ofa combustion flame core is prevented. Accordingly, due to the reductionof flame transmitting speed, a misfire or change of combustion mayoccur.

U.S. Pat. No. 4,543,928 to von Seggern discloses a two-cycle engine inwhich an air swirl is formed about a cylinder axis and stratification isobtained between the mixture in the ignition region and the air in thepiston region. However, this swirl is not an exhaust gas swirl and nostratification is obtained between the exhaust gas and the fresh air.

SUMMARY OF THE INVENTION

An object of this invention is to provide a two-cycle engine, in whichan intake port and an exhaust port provided in a cylinder head areopened to a combustion chamber, capable of attaining a stratificationbetween fresh air and exhaust gas by collecting the fresh air around thecylinder head (a spark plug), especially in an idling or light loadcondition, so that it will easily burn even when only a small amount offuel is present.

Another object of this invention is to provide a two-cycle engine havingthree, or a multiple of three, cylinders, wherein an intake port and anexhaust port provided in a cylinder head are opened to a combustionchamber, capable of improving output power in a heavy load condition byusing charging effects due to an exhaust pulsation between cylinders.

A two-cycle internal combustion engine comprises: a cylinder head havingat least one intake port for introducing fresh air into a combustionchamber and at least one exhaust port for discharging exhaust gas fromthe combustion chamber; an air charging means for supplying compressedfresh air to the intake port; intake and exhaust valves for opening andclosing the intake and exhaust ports, respectively; and a valveoperating means operated in response to crank angle.

According to this invention the two-cycle engine further comprises: anexhaust system having a means for substantially restraining a pulsationpressure of exhaust gas in the exhaust port during at least an idling orlight load running condition of the engine; the valve operating meansincluding a means for opening the exhaust valve earlier than the intakevalve when the speed of a downward movement of the piston is relativelyhigh, such that a part of the exhaust gas in the exhaust port flows backto the combustion chamber in the idling or light load condition; and,means for forming an exhaust gas swirl rotating around a cylinder axisof the combustion chamber when the exhaust gas flows back to thecombustion chamber; the valve operating means further including a meansfor opening the intake valve so as to slowly introduce fresh air to theexhaust gas swirl in the idling or light load condition.

In another aspect of this invention, the two-cycle engine furthercomprises intake ports including a first intake port for directing thefresh air widely into the combustion chamber and a second intake portdirecting the fresh air downward into the combustion chamber along thecylinder wall thereof; a fuel injector provided in at least the firstintake port for injecting fuel toward an ignition spark plug in thecombustion chamber; an inlet air control valve for substantially closingthe second intake port during an idling or light load running conditionof the engine; the valve operating means including a means for openingthe exhaust valve earlier than the intake valves when the speed of adownward movement of the piston is relatively high, such that a part ofthe exhaust gas in the exhaust port flows back to the combustion chamberin the idling or light load condition; an exhaust system having a meansfor substantially restraining a pulsation pressure of the exhaust gas inthe exhaust port during at least the idling or light load runningcondition of the engine; means for forming an exhaust gas swirl rotatingaround a cylinder axis of the combustion chamber when the exhaust gasflows back to the combustion chamber; the valve operating means furtherincluding a means for opening the intake valves so as to slowlyintroduce fresh air to the exhaust gas swirl through the first intakeport in the idling or light load condition.

According to the present invention, in an idling or light loadcondition, fresh air is collected around the cylinder head having aspark plug, and a suitable stratification can be obtained between lowerexhaust gas and upper fresh air, which is heated and activated by theheat of adjacent exhaust gas, so that easy burning can be achieved.

A two-cycle internal combustion engine having three, or a multiple ofthree, further comprises: an exhaust system having an exhaust controlvalve for substantially restraining a pulsation pressure of the exhaustgas in the exhaust port in an idling or light load running condition ofthe engine and, further, for effecting air charging due to exhaust gaspulsation between the cylinders in a high load condition; the valveoperating means including a means for opening the exhaust valve earlierthan the intake valve when the speed of a downward movement of thepiston is relatively high; and, means for forming a swirl of exhaust gasor fresh air rotating around a cylinder axis of the combustion chamberwhen exhaust gas or fresh air once accumulated in the exhaust port flowsback to the combustion chamber, respectively.

Another aspect of a two-cycle engine having three, or a multiple ofthree, cylinders further comprises: a valve operating means including ameans for opening the exhaust valve earlier than the intake valve whenthe speed of a downward movement of the piston is relatively high; anintake and exhaust system for effecting cross-scavenging and aircharging due to exhaust gas pulsation between the cylinders during atleast a high load running condition of the engine; and, means forforming a swirl of fresh air rotating around a cylinder axis of thecombustion chamber when the fresh air once accumulated in the exhaustport due to the pulsating air charging effects flows back to thecombustion chamber.

In a two-cycle engine having three, or a multiple of three, cylinders,according to the present invention, charging effects due to exhaustpulsation between cylinders and cross-scavenging can be obtained in aheavy load condition, so that output power in that condition can beimproved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a six-cylinder two-cycle internalcombustion engine according to the present invention;

FIG. 2 is a schematic view illustrating a main part of the two-cycleengine shown in FIG. 1;

FIG. 3 is a cross-sectional view illustrating a main part of thetwo-cycle engine shown in FIG. 1;

FIGS. 4 and 5 are schematic views illustrating fresh air flowing into acombustion chamber through two intake valves, respectively;

FIG. 6 is a timing chart illustrating the opening and closing of exhaustand intake valves and the injection timing by fuel injectors;

FIG. 7 is an opening and closing timing chart of exhaust valves in aplurality of cylinders;

FIGS. 8a-8e are views for explaining, in series, the changes of exhaustgas and fresh air in an idling or light load running condition of theengine;

FIGS. 9a-9e are views for explaining, in series, the changes of exhaustgas and fresh air in a heavy load condition;

FIG. 10 is a diagram illustrating the relationship between engine loadand charging effect due to exhaust pulsation;

FIG. 11 is a schematic view illustrating exhaust gas swirl rotatingaround an inclined axis;

FIG. 12 is a schematic plan view illustrating an embodiment in whichboth exhaust ports are constituted as eccentric ports;

FIG. 13 is an enlarged cross-sectional view taken along line XIII--XIIIin FIG. 2;

FIG. 14 is an enlarged cross-sectional view taken along line XIV--XIV inFIG. 2;

FIG. 15 is a timing chart illustrating the opening and closing ofexhaust and intake valves and the fuel injection in another embodiment;

FIG. 16 is a schematic plan view illustrating exhaust passages in aV-shaped six-cylinder two-cycle engine;

FIG. 17 is a schematic view of an exhaust gas control system in athree-cylinder two-cycle engine;

FIG. 18 is an opening and closing timing chart of exhaust valves in thethree-cylinder engine shown in FIG. 17;

FIG. 19 is a schematic view of an exhaust system in anotherthree-cylinder engine;

FIGS. 20 and 21 are schematic views of exhaust systems in asingle-cylinder engine;

FIG. 22 is a schematic view of a two-cycle diesel engine according tothe present invention; and,

FIG. 23 is a cross-sectional view of a main part of the two-cycle dieselengine shown in FIG. 22.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIGS. 1, 2, and 3, reference numeral 1 denotes anengine body; 10, an air intake system; and 30, an exhaust system. In theengine body 1, reference numeral 2 denotes a combustion chamber(cylinder); 3, a piston; 4, an ignition spark plug; 5, maskings; 6, acylinder head; and 7, a cylinder block. In the intake system 10,reference numeral 11 denotes an air cleaner; 12, an air flow meter; 13,a throttle valve; 14, a mechanical supercharger; 15, an intercooler; 16,an inlet air control valve; 17a and 17b, surge tanks; 18a and 18b, reedvalves; 19a and 19b, fuel injectors; 20a and 20b, intake ports; and 21aand 21b, intake valves. In the exhaust system 30, reference numerals 31aand 31b denote exhaust valves; 32a and 32b, exhaust ports; 33, anexhaust manifold; 34, an exhaust control valve; 35, a catalyzer; and 36,a muffler.

The inlet air flows through the air cleaner 11 and is regulated by thethrottle valve 13. The air flow meter 12 is provided between the aircleaner 11 and the throttle valve 13 and meters the flow of inlet air.Located downstream of the throttle valve 13 is a mechanical supercharger14, which compresses the inlet air. The inlet air heated by thesupercharger 14 is then cooled by the intercooler 15 disposed downstreamthereof, to increase the volumetric efficiency of the inlet air. Themechanical supercharger 14 may be, for example, a Roots pump typesupercharger including a housing in which a pumping operation is carriedout to compress the inlet air. The supercharger 14 is, as well known,connected to a crankshaft of the engine by pulleys and a belt (not shownin the drawings), to rotate at a speed corresponding to the enginerevolutional speed. A vane-pump or the like also may be used in place ofthe above-mentioned Roots pump 14.

Downstream of the intercooler 15, the intake system 10 is divided intotwo inlet passages, i.e., an inlet passage 10a for a light load andanother inlet passage 10b for a heavy load. The passage 10b is providedwith the inlet air control valve 16, which may be a general butterflyvalve closed during the idling or light load engine condition and openedduring a heavy load (including a middle load) condition. The inletpassages 10a and 10b are connected to the surge tanks 17a and 17b,respectively. The downstream sides thereof are divided by manifolds andled to the respective cylinders and connected to the respectivecombustion chambers via the intake ports 20a and 20b formed in thecylinder head 6. These intake ports 20a and 20b are directly opened tothe combustion chamber 2 from the cylinder head 6. Fuel injectors 19aand 19b are provided in the inlet ports 20a and 20b, respectively, ineach cylinder. The one-way valves 18a and 18b comprising reed valves areprovided upstream of the fuel injectors 19a and 19b.

One of the fuel injectors 19a injects fuel toward the region of a sparkplug 4 in any engine condition, and the other fuel injector 19b injectsfuel toward the center of the combustion chamber 2 during a heavy loadcondition wherein the inlet air control valve 16 is opened. Therefore,the fuel injector 19b has a large-sized injection nozzle and injectsmore fuel than the fuel injector 19a. The inlet air mixed with injectedfuel flows into the combustion chamber 2 via the intake ports 20a and20b provided with the poppet type intake valves 21a and 21b,respectively. These intake valves 21a and 21b are opened or closed in atiming synchronized with the crank angle, as will be mentioned later.

The air/fuel mixture flows from the intake ports 20a and 20b to thecombustion chamber 2, as shown in FIGS. 4 and 5. That is, the air/fuelmixture flows from the intake port 20a along substantially the wholeperiphery of the bevel face of the intake valve 20aas shown in FIG. 4.On the other hand, as shown in FIG. 5, the air/fuel mixture flows fromthe intake port 20b along a part of the periphery of the bevel face ofthe intake valve 20b, i.e., a region of the cylinder wall, and straightdownward along the cylinder wall. To this end, a masking wall 22, forexample, as shown by a dotted-line in FIGS. 3 and 5, is formed on theinner wall of the intake port 20b in the vicinity of the intake valve21b and near to the center of the cylinder. Therefore, a large amount ofthe air/fuel mixture (fresh air) flows through the intake port 20b at ahigh speed and along the inclined face of the masking wall 22, so thatthe air/fuel mixture is directed to the side of cylinder wall anddownward to the combustion chamber 2 along the cylinder wall.

Two exhaust ports 32a and 32b are also directly opened to the combustionchamber 2 from the cylinder head 6 at positions opposite to the intakeports 20a and 20b, respectively. These exhaust ports 32a and 32b arealso opened or closed by the poppet-type exhaust valves 31a and 31b,respectively, in a timing synchronized with the crank angle, as will bementioned later. The exhaust ports 32a and 32b are combined andconnected to an exhaust manifold 33 at the downward side and in thevicinity of the exhaust valves 31a and 31b. The exhaust manifold 33 isprovided with an exhaust control valve 34 which interrupts or connectsthe flow between the manifold pipes of Nos. 1 to 3 cylinders and themanifold pipes of Nos. 4 to 6 cylinders. Two manifold portions 33a and33b are connected by a two-way manifold 37, which is connected to anexhaust pipe 38.

In the illustrated six-cylinder two-cycle engine, assuming that thecycle is repeated in the order 1, 6, 2, 4, 3, 5 at each 60° crank angle,the operation cycle is repeated at each 120° crank angle in eachcylinder group 1, 2, and 3, or 4, 5, and 6. A catalyzer 35 and a muffler36 are located downstream of the exhaust pipe 38. The exhaust controlvalve 34 serves merely to control the exhaust gas pressure and,therefore, a general butterfly valve may be sufficient for that purpose,since strict sealing is not required. However, it is understood that apoppet-type valve or the like having a high sealing effect may be used.The exhaust control valve 34 is controlled in such a manner that itopens in the idling or light load running condition and closes in thehigh load running condition.

The combustion chamber 2 in each cylinder is defined between thecylinder head 6, the piston 3, and the cylinder block 7 and is providedwith an ignition spark plug 4 at the center of the cylinder head 6.Around the exhaust ports 32a and 32b, the cylinder head 6 is providedwith masking portions 5, which facilitate the formation of a swirl inthe exhaust gas, particularly an appropriate swirl rotating around theaxis of the cylinder (combustion chamber 2) when the exhaust gas flowsback to the combustion chamber 2 from the exhaust ports 32a and 32b viathe bevel faces of the exhaust valves 31a and 31b, especially in theidle or light load running condition. In place of (or in addition to)such masking portions 5, one or both of the exhaust ports 32a and 32bmay be an eccentric exhaust port, as shown at 32b in FIG. 2, so that aswirl is formed in the exhaust gas when it flows in the tangentialdirection back to the combustion chamber 2 from the exhaust port 32b.

The intake valves 21a and 21b and the exhaust valves 31a and 31b areoperated, although not illustrated, by cam members mounted on a camshaft which is rotated at the same speed as the crank shaft, so thatthese valves are opened and closed in accordance with predeterminedtimings, as shown in FIG. 6. That is, the exhaust valves 31a and 31b arefirst opened at approximately -125° with respect to the bottom deadcenter (BDC) and then the intake valves 21a and 21b are opened atapproximately -90°. On the other hand, the exhaust valves 31a and 31bare closed at approximately +40° with respect to the bottom dead center(BDC) and the intake valves 21a and 21b are closed at approximately+60°. The fuel injectors 19a and 19b inject fuel approximately between+45° to -40°.

In the illustrated six-cylinder two-cycle engine, assuming that theoperation cycle is repeated in the order 1, 6, 2, 4, 3, 5 at each 60°crank angle, the exhaust valves 31a and 31b of each cylinder are openedand closed as illustrated in FIG. 7. The solid lines in FIG. 7 indicatetime periods, with respect to the crank angle of the cylinder No. 1,during which the exhaust valves 31a and 31b in the respective cylindersare opened. On the other hand, the exhaust control valve 34 iscontrolled in such a manner that it opens at least in the idling orlight load running condition, as mentioned above.

Therefore, in the idling or light load running conditions, all thebranches of the exhaust manifold 33 are connected to each other. Forexample, as illustrated in FIG. 7, in the initial period K in thecylinder No. 1 during which the exhaust valves 31a and 31b start toopen, the exhaust valves in the cylinder No. 3 are still open. In theintermediate period L, the exhaust valves in cylinder No. 6 start toopen, and in the final period M, the exhaust valves in the cylinder No.2 start to open. In particular, due to the exhaust pressure from theother cylinder group (i.e., cylinder No. 6), the exhaust ports 32a and32b are always subjected to a substantial positive pressure and,therefore, the effects of the exhaust pulsation charge in each cylinderare not generated. In the other cylinders, the same operation isperformed so that the exhaust pressure in each cylinder cooperativelyinterferes with each other pressure and controls the back pressure, asmentioned later.

On the other hand, in the heavy load running condition of the engine,the exhaust control valve 34 is closed so that the exhaust pressure issubjected only slightly to the back pressure from the cylinder No. 6.Therefore, the exhaust ports 32a and 32b in the cylinder No. 1 aresubjected to pressure interference (M) from the cylinder No. 2 and,therefore, the effects of the exhaust pulsation charge are generated. Toavoid the pulsation pressure which would be generated just after ablowdown in a low speed running condition, a resonance chamber (notshown) may be connected to the exhaust port.

The operation of a six-cylinder two-cylinder engine according to thisinvention will now be described with reference to FIGS. 6 through 9.

First, in the idling or light load running condition of the engine, theinlet air control valve 16 is closed and the exhaust control valve 34 isopened. During the downward movement of the piston 3, the exhaust valves31a and 31b start to open when the piston 3 arrives at a pointapproximately -125° from the top dead center (BDC) in FIG. 6. Therefore,at a region (A) in FIG. 6, exhaust gas flows out through the exhaustvalves 31a and 31b which are just opened (weak blowdown P in FIG. 8).This blowdown (P) is quickly completed, since in the idling or lightload condition, the pressure in the combustion chamber 2 is low and theamount of exhaust gas is small. That is, in the exhaust ports 32a and32b, although the exhaust gas pressure temporarily rises to 2 to 3kg/cm², it is immediately reduced to and balanced at about 1.05 kg/cm².When the engine is running at a high speed, the exhaust gas pressure ismore stably balanced.

Then, at a point (B), i.e., crank angle -90° in FIG. 6, the speed of thedownward movement of the piston 3 is relatively high. Therefore, thecylinder pressure becomes a vacuum and is influenced by the exhaust gaspressure of the other cylinder group (i.e., cylinder No. 6), as shown atL in FIG. 7. Therefore, high temperature exhaust gas which has been once"blowndown" to the exhaust ports 32a and 32b flows back to thecombustion chamber 2, as shown at Q in FIG. 8. An exhaust gas swirl (R)is formed around the axis of the cylinder chamber 2 by a swirl formingmeans, i.e., the eccentric port 32b and the maskings 5. This swirl isrelatively slow and serves to prevent an escape of the exhaust gas heatin the combustion chamber 2.

Fresh air does not flow in at the point just after the intake valves 21aand 21b are opened, since the pressure in the intake port 20a iscontrolled by the throttle valve 13 and the lift of the intake valve 21ais so small that the port is further throttled. Therefore, the flow backof the exhaust gas from the exhaust ports 32a and 32b proceeds further.When the piston 3 moves further downward and the speed thereof becomesslow, the lift of the intake valves is increased, as shown at C in FIGS.6 and 8, and the fresh air (mixture), which has been controlled by thethrottle valve 13 and compressed at a low pressure by the supercharger14, flows into the combustion chamber 2 via the intake valve 21a in theintake port 20a .

In this case, as mentioned above, the fresh air flows from the intakeport 20a along substantially the whole periphery of the bevel face ofthe intake valve 20a, as shown in FIG. 4. In this running condition, thespeed of the downward movement of the piston 3 is relatively slow, thepressure reduction in the cylinder is small and, therefore, the velocityof the fresh air is low. Therefore, the fresh air flows slowly onto theexhaust swirl (R) in the combustion chamber 2 and is collected at theupper portion thereof, i.e., around the spark plug 4 opposite to thecylinder head 6. Thus, a suitable stratification consisting of a freshair region (S) at the cylinder head (upper) side and an exhaust gasregion (R) at the piston (lower) side is obtained in the combustionchamber 2. As the exhaust gas (R) swirls around the cylinder axis, thisstratification of the fresh air (S) and exhaust gas (R) is maintaineduntil the piston 3 arrives at the bottom dead center (BDC), as shown atD in FIGS. 6 and 8.

Even after the intake valve 21a is closed and the fresh air intake issubstantially ended, as shown at E in FIGS. 6 and 8, the stratificationof the fresh air (S) and exhaust gas (R) is still maintained. As thisstratification is also maintained until the end of the pressurizingstroke, the fresh air in the vicinity of the cylinder head 6 is heatedand activated by the high temperature exhaust gas region near the piston3. Therefore, if this engine is in the idling condition, the mixture isreadily burnt by the spark plug 4 at the end of the pressurizing strokeand the flame spreads so that a stable combustion can be attained. Ifthe engine is in the light load running condition after the engine iswarmed up, the temperature of the exhaust gas in the cylinder chamber ishigh and activation of the fresh air is promoted, so that it is possiblefor the fresh air to self-ignite without ignition by the spark plug 4,due to the adiabatic compression during the pressurizing stroke.

At points D and E, as illustrated at L and M in FIG. 7, the pressure inthe exhaust ports 32a and 32b is always positive in the idling or lightload running condition due to the exhaust gas (back) pressure from theother cylinders exerted thereon. Therefore, there is substantially nocharging effect due to exhaust pulsation, so that the fresh air isprevented from flowing out to the exhaust system and the swirl flow (R)is not disturbed. Therefore, a stable stratification combustion can beattained.

As mentioned above, in the idling or light load running condition, asshown in FIG. 10, the exhaust pulsation charging effect is prevented byclosing the exhaust control valve 34, and an exhaust swirl is generateddue to the exhaust gas blowdown. On the other hand, the fresh air isintroduced to the combustion chamber in the vicinity of cylinder head 6through the intake port 20a by closing the intake air control valve 16.Therefore, a stratification of the fresh air and exhaust gas swirl iscreated, and accordingly, in the warming-up or idling condition, astable combustion can be obtained with ignition by the spark plug 4. Inthe after warmed-up and light load condition, as the exhaust gas is at ahigh temperature, it is possible for the fresh air to self-ignitewithout ignition by the spark plug 4.

In the heavy load running condition of the engine, the intake aircontrol valve 16 is opened and the exhaust control valve 34 is closed.During the downward movement of the piston 3, the exhaust valves 31a and31b start to open at the point (a) in FIG. 6, as shown in FIG. 9A. Theexhaust gas flows out abruptly (blowdown P) through the exhaust valves31a and 31b which are just opened. The amount of exhaust gas in theheavy load condition is so large that the blowdown (P) is strong andcontinues for a relatively long time. The blowdown (P) is completed at acrank angle of -90°. Therefore, a large amount of exhaust gas isdischarged. At a point (b) in FIG. 6, i.e., at a crank angle of -80°,the intake valves 21a and 21b are substantially opened to introduce thefresh air (T), as shown in FIG. 9B. Therefore, the compressed fresh air(air-fuel mixture) starts to flow into the combustion chamber 2 throughthe intake ports 20a and 20b via the intake valves 21a and 21b.

In the heavy load condition, as the inlet air control valve 16 is openedas mentioned above, the fresh air flows through both the intake ports20a and 20b. Especially, a large amount of fresh air flows rapidlythrough the intake port 21b into the combustion chamber 2 directlydownward along the cylinder wall, as shown in FIG. 5. Therefore, asshown in FIG. 9B, a so-called cross-scavenging starts between theexhaust gas (U) and the fresh air (T). Then, as shown in FIGS. 6(b) and(c) and FIGS. 9B and 9C (during a crank angle of from -80° to -50°), thepressure in the exhaust ports 32a and 32b becomes temporarily a vacuumdue to the charging effects of exhaust gas pulsation caused by thestrong blowdown. Therefore, the introduction of fresh air is furtherpromoted and a part of the fresh air (V) is temporarily accumulated inthe exhaust ports 32a and 32b and the exhaust manifold 33.

Then, at a point (d) in FIG. 6(b) and FIG. 9D, a strong positivepressure as shown at M in FIG. 7 is exerted due to a strong blowdown ofthe other cylinder (the second cylinder) in which the exhaust valvesthereof are just opened, so that the fresh air (V) which has beentemporarily accumulated in the exhaust ports 32a and 32b and the exhaustmanifold 33 now flows back to the combustion chamber 2. While the freshair flows back through the eccentric exhaust port 32b and masking 5 intothe combustion chamber 2, it forms a strong swirl (X). After the intakevalves 21a and 21b are closed, as shown at (e) in FIG. 6 and FIG. 9E,the fresh air no longer flows through the exhaust ports 32a and 32b.

As mentioned above, in the heavy load condition, the inlet air controlvalve 16 is opened to rapidly introduce a large amount of fresh airdownward along the cylinder wall of the combustion chamber 2, so that aso-called cross scavenging can be achieved. On the other hand, theexhaust control valve 34 is closed so as to generate a positive/negativeexhaust gas pulsation, as shown in FIG. 10. This effect of exhaustpulsation among the cylinders promotes the introduction of the freshair. That is, a part of the fresh air which has been once accumulated inthe exhaust ports and the exhaust manifold flows back to the cylinder,so that a large amount of fresh air can be supplied and a strong swirlis created, whereby transmission of the combustion flame is improved.

As mentioned above, when the exhaust gas flows back to the combustionchamber 2, a swirl is formed by means of the maskings 5 on the wall ofthe cylinder head and/or the eccentric port 32b. It may be desirable toform such a swirl rotating around an axis Y inclined from the verticalaxis toward the intake valves, as shown in FIG. 11, by changing theshape of the maskings 5, for example. In this case, only a slightdynamic pressure is exerted on the intake valve (port) and, therefore,the fresh air is further prevented from flowing through the exhaustsystem. In any case, it is sufficient to ensure that the stratificationis not disturbed but maintained between the exhaust gas swirl thusformed and the upper fresh air region. Therefore, the meaning of "aroundthe cylinder axis" should be widely interpreted.

In order to form an exhaust gas swirl rotating around the cylinder axis,both of the exhaust ports 32a and 32b may be constituted as eccentricports which are arranged in the direction perpendicular to a line alongwhich the respective cylinders are aligned, as shown in FIG. 12. In thiscase, both of the intake ports 32a and 32b are also arranged in thedirection perpendicular to the line along which the cylinders arealigned.

The maskings 5 formed on the wall of the cylinder head 6 may bedesirably constituted as maskings 5a and 5b, as shown in FIGS. 13 and14. The masking 5a is formed between the intake valve 21a and theexhaust valve 31a and has a shape such that fresh air flowing from theintake port 20a into the combustion chamber 2 is prevented from flowingthrough the exhaust port 32a. On the other hand, the masking 5b isformed between the intake valve 21b and the exhaust valve 31b and has ashape such that exhaust gas or fresh air flowing from the exhaust port32b back to the combustion chamber 2 is prevented from flowing throughthe intake port 20b.

In the above embodiment, as shown in FIG. 6, the exhaust valves 31a and31b are closed earlier than the intake valves 21a and 21b. However, asshown in FIG. 15, the intake valves 21a and 21b may be closed earlier ata crank angle of +40°, so that the intake valves 21a and 21b are alreadyclosed when the fresh air flows from the exhaust valves 31a and 31b backto the combustion chamber 2, to prevent the fresh air from flowing backthrough the intake ports 20a and 20b and to increase the realcompression ratio of the fresh air in the cylinder chamber 2.

If this six-cylinder two-cycle engine is constituted as a so-calledV-shaped engine, as shown in FIG. 16, exhaust pipes 52 connected toexhaust ports 51 of the respective cylinders are connected to a bypasspassage 53 which can be separated by means of an exhaust control valve34 into two cylinder groups of Nos. 1 through 3 and Nos. 4 through 6.The bypass passage 53 is connected at the downstream side thereof to asingle exhaust pipe 38. In this case, the three exhaust pipes 52 in thesame cylinder group can be arranged to each other, so that the exhaustpipes or the like can be more easily arranged and the exhaust pulsationcan be more effectively used.

Although the above mentioned two-cycle engine has six-cylinders, atwo-cycle engine according to the present invention may have threecylinders, or one or two cylinders.

In a three-cylinder engine, as shown in FIG. 17, exhaust pipes 41 of therespective cylinders are arranged independently to each other and bypassvalves 42 are provided to mutually connect these exhaust pipes 41. Inthe idling or light load running condition of the engine, these bypassvalves 42 are closed, so that the effective length of each exhaust pipe41 becomes longer and an exhaust pulsation is not exerted on the exhaustpipes 41 of the other cylinders. Therefore, in this condition, the back(exhaust gas) pressure is always substantially positive. In the heavyload condition, these bypass valves 42 are opened, so that each exhaustpipe 41 is subjected to a blowdown M of back pressure in the othercylinder having a cycle delayed by 120°, as shown in FIG. 18 and,therefore, charging effects due to exhaust pulsation, as mentionedabove, can be expected.

In another three-cylinder engine, as shown in FIG. 19, exhaust pipes 61of the respective cylinders are connected to a single pipe. Therefore,according to this embodiment, in the heavy load condition, each exhaustpipe 61 is subjected to a blowdown of back pressure in the othercylinder having a cycle delayed by 120° and, therefore, exhaustpulsation charging effects also can be expected, in the same manner asmentioned above.

In a single-cylinder engine, as shown in FIGS. 20 or 21, an exhaust pipe43 is provided with an exhaust control valve 45 and a passage 44 forbypassing the valve 45. In the idling or light load condition, the valve45 is closed, so that the effective length of the exhaust pipe 43becomes substantially longer and, therefore, the back (exhaust gas)pressure is always substantially positive in this condition. In theheavy load condition, the valve 45 is opened, so that effective lengthof the exhaust pipe 43 becomes substantially shorter. Reference numeral46 indicates a portion opened to the atmosphere; and 47, a volume (FIG.20). In a one or two-cylinder engine, however, the charging effects ofexhaust pulsation cannot be expected.

A two-cycle engine according to the present invention also may beconstituted as a diesel engine, as shown in FIGS. 22 and 23. In thiscase, a stratification can be also obtained between the fresh air beforefuel injection and the exhaust gas. After the fresh air is sufficientlyheated by the exhaust gas, fuel is injected directly into the combustionchamber and, therefore, it is possible that the fresh air willself-ignite without ignition by a spark plug even at a relatively lowcompression ratio. FIGS. 22 and 23 are views similar to FIGS. 1 and 3,respectively, in which the same or corresponding parts as those in FIGS.1 and 3 are indicated by the same reference numerals. However, in thistwo-cycle diesel engine, a single fuel injector 19 is provided in placeof a spark plug 4 (FIG. 3) to directly inject fuel into the combustionchamber 2. Also, the exhaust system is divided into two groups 70a and70b for the cylinder Nos. 1, 2, and 3, cylinder Nos. 4, 5, and 6,respectively. A particle trapper or fixed-throttle 71 and a muffer 36are provided independently in each of the exhaust systems 70a and 70b.

We claim:
 1. A two-cycle internal combustion engine comprising: acylinder head having at least one intake port for introducing fresh airinto a combustion chamber and at least one exhaust port for dischargingexhaust gas from said combustion chamber; an air charging means forsupplying compressed fresh air to said intake port; intake and exhaustvalves for opening and closing said intake and exhaust ports,respectively; and a valve operating means operated in response to acrank angle; characterized in that said engine further comprises:anexhaust system having a means for substantially restraining a pulsationpressure of exhaust gas in said exhaust port during at least an idlingor light load running condition of the engine; said valve operatingmeans including a means for opening said exhaust valve earlier than saidintake valve when a speed of a downward movement of a piston isrelatively high, such that a part of exhaust gas in said exhaust portflows back to said combustion chamber in the idling or light loadconditions; and, means for forming an exhaust gas swirl rotating arounda cylinder axis of the combustion chamber when the exhaust gas flowsback to the combustion chamber; said valve operating means furtherincluding a means for opening said intake valve so as to slowlyintroduce fresh air onto said exhaust gas swirl in the idling or lightload condition.
 2. A two-cycle engine as set forth in claim 1, whereinsaid means for forming an exhaust gas swirl comprises a masking formedon a cylinder head inner wall of the combustion chamber and around saidexhaust port.
 3. A two-cycle engine as set forth in claim 1, whereinsaid means for forming an exhaust gas swirl comprises an eccentricexhaust port.
 4. A two-cycle internal combustion engine comprising: acylinder head having intake ports for introducing fresh air into acombustion chamber and at least one exhaust port for discharging exhaustgas from said combustion chamber; an air charging means for supplyingcompressed fresh air to said intake ports; intake and exhaust valves foropening and closing said intake and exhaust ports, respectively; and avalve operating means operated in response to a crank angle;characterized in that said engine further comprises:said intake portsincluding a first intake port for directing the fresh air widely intothe combustion chamber, and a second intake port directing the fresh airdownward into the combustion chamber along the cylinder wall thereof; afuel injector provided in at least said first intake port for injectingfuel toward an ignition spark plug in the combustion chamber; an inletair control valve for substantially closing said second intake portduring an idling or light load running condition of the engine; saidvalve operating means including a means for opening said exhaust valveearlier than said intake valves when the speed of the downward movementof the piston is relatively high, such that a part of exhaust gas insaid exhaust port flows back to said combustion chamber in the idling orlight load condition; an exhaust system having a means for substantiallyrestraining a pulsation pressure of exhaust gas in said exhaust portduring at least the idling or light load running condition of theengine; means for forming an exhaust gas swirl rotating around acylinder axis of the combustion chamber when the exhaust gas flows backto the combustion chamber; said valve operating means further includinga means for opening said intake valves so as to slowly introduce freshair onto said exhaust gas swirl through said first intake port in theidling or light load condition.
 5. A two-cycle internal combustionengine having three, or a multiple of three, cylinders comprising: acylinder head having for each cylinder at least one intake port forintroducing fresh air into a combustion chamber and at least one exhaustport for discharging exhaust gas; an air charging means for supplyingcompressed fresh air to said intake port; intake and exhaust valves foropening and closing said intake and exhaust ports, respectively; and avalve operating means operated in response to a crank angle;characterized in that said engine further comprises:an exhaust systemhaving an exhaust control valve for substantially restraining apulsation pressure of exhaust gas in said exhaust port in an idling orlight load running condition of the engine, and further, effecting aircharging due to exhaust gas pulsation between cylinders in a high loadcondition; said valve operating means including a means for opening saidexhaust valve earlier than said intake valve when the speed of thedownward movement of the piston is relatively high; and, means forforming a swirl of exhaust gas or fresh air rotating around a cylinderaxis of the combustion chamber when exhaust gas or fresh air onceaccumulated in said exhaust port flows back to the combustion chamber,respectively.
 6. A two-cycle engine as set forth in claim 5, whereinsaid means for forming an exhaust gas swirl comprises a masking formedon a cylinder head inner wall of the combustion chamber and around saidexhaust port.
 7. A two-cycle engine as set forth in claim 5, whereinsaid means for forming an exhaust gas swirl comprises an eccentricexhaust port.
 8. A two-cycle internal combustion engine having three, ora multiple of three, cylinders comprising: a cylinder head having foreach cylinder at least one intake port for introducing fresh air into acombustion chamber and at least one exhaust port for discharging exhaustgas; an air charging means for supplying compressed fresh air to saidintake port; intake and exhaust valves for opening and closing saidintake and exhaust ports, respectively; and a valve operating meansoperated in response to a crank angle; characterized in the said enginefurther comprises:said valve operating means including a means foropening said exhaust valve earlier than said intake valve when the speedof the downward movement of the piston is relatively high; an intake andexhaust system for effecting cross-scavenging and air charging due toexhaust gas pulsation between cylinders during at least a high loadrunning condition of the engine; and, means for forming a swirl of freshair rotating around a cylinder axis of the combustion chamber when thefresh air once accumulated in said exhaust port due to the pulsed aircharging effects flows back to the combustion chamber.